predefined_variables.md: Clarify CI_COMMIT_REF_PROTECTED an always present true/false value

Unclear for me as it was in https://stackoverflow.com/a/59023344

doc/ci/variables/predefined_variables.md

@@ -33,7 +33,7 @@ future GitLab releases.**
 | `CI_COMMIT_DESCRIPTION`                       | 10.8   | all    | The description of the commit: the message without first line, if the title is shorter than 100 characters; full message in other case.                                                                                                                                                                                                                    |
 | `CI_COMMIT_MESSAGE`                           | 10.8   | all    | The full commit message.                                                                                                                                                                                                                                                                                                                                   |
 | `CI_COMMIT_REF_NAME`                          | 9.0    | all    | The branch or tag name for which project is built                                                                                                                                                                                                                                                                                                          |
-| `CI_COMMIT_REF_PROTECTED`                     | 11.11  | all    | If the job is running on a protected branch                                                                                                                                                                                                                                                                                                                |
+| `CI_COMMIT_REF_PROTECTED`                     | 11.11  | all    | `true` if the job is running on a protected branch, `false` if not                                                                                                                                                                                                                                                                                                               |
 | `CI_COMMIT_REF_SLUG`                          | 9.0    | all    | `$CI_COMMIT_REF_NAME` lowercased, shortened to 63 bytes, and with everything except `0-9` and `a-z` replaced with `-`. No leading / trailing `-`. Use in URLs, host names and domain names.                                                                                                                                                                |
 | `CI_COMMIT_SHA`                               | 9.0    | all    | The commit revision for which project is built                                                                                                                                                                                                                                                                                                             |
 | `CI_COMMIT_SHORT_SHA`                         | 11.7   | all    | The first eight characters of `CI_COMMIT_SHA`                                                                                                                                                                                                                                                                                                              |